Contemporary vehicles including passenger cars, light duty trucks, and medium duty vehicles use evaporative emissions systems to prevent unnecessary emission of hydrocarbon (HC) vapors into the atmosphere. These emissions are primarily composed of gasoline vapors leaking from a vehicle's fuel tank to the air. In a typical system, the fuel tank is periodically vented into a canister filled with charcoal that filters the HC vapors and releases the filtered air to the atmosphere. The charcoal traps the hydrocarbon molecules from the polluting vapors, preventing them from leaking to the atmosphere.
Near term regulation requires the monitoring of the vehicle's evaporative emission system to ensure integrity of operation. This requirement specifies, among other things, checking of the absence of leaks in the system. More specifically, the California Air Resources Board, CARB, specifies in their proposed On Board Diagnostic II, or OBD II, requirement to check the evaporative emissions system for leaks. This requires detecting system leaks equivalent to an office larger than 0.020 inches in diameter for vehicles produced in model year 2000.
A prior art for scheme for detecting leaks measures a re-pressurization time. In this case the evaporative emissions system's vent to atmosphere is closed, and then a weak vacuum (DP=10 inches of water) is drawn on the fuel tank--waiting to see how long the fuel tank takes to re-pressurize. If the fuel tank takes a long time to re-pressurize, then there are no significant leaks. If the fuel tank re-pressurizes quickly then a significant leak is indicated. There are several problems with this scheme including a length of the re-pressurization time. With a relatively small leak and a nearly empty tank, the re-pressurization time can be unacceptably long. Furthermore, the amount of space above the fuel varies with fuel level, and the corresponding change in fuel tank volume causes the re-pressurization time to vary as a function of fuel level. Also, inaccuracies can be caused by fuel evaporation during the leak test. With high volatility (winter) fuel, on a warm day fuel evaporation generates vapor at a rate that exceeds the amount of gas flow generated by 10 inches of water across a 0.020 inch hole. This can cause false leak detection on warm days when the fuel tank contains high volatility fuel. Additionally, in some fuel tanks, the 10 inches of water vacuum drawn on the fuel tank during the test causes the fuel tank to flex. This causes the fuel tank to hold vacuum for a longer period of time during re-pressurization.
Another prior art scheme uses a positive displacement pump to pressurize the fuel tank with air and measure the air flow with the fuel tank pressurized. Safety is at issue here because the air pumped into the fuel tank may cause an explosion hazard. This scheme is subject to the same problems with high volatility fuel on warm days as other prior art schemes and has the potential of increasing HC emissions during a test of a leaky system.
What is needed is an improved method of detecting a leak in an evaporative emissions system for a vehicle that is safer and more accurate than prior art schemes.